EFFECTS UPON GERMINATION 803 



if accompanied by intermittent temperatures and light. Maier (71) 

 found acids favorable. There has been considerable disagreement 

 about the best substratum to use in combination with intermittent 

 temperatures, light, or nitrates in testing Poa achenes. Pieper (85) 

 found blotting paper or sand with 60 per cent moisture capacity best 

 for Poa pratensis. Laschke (59) used clay, sand, or wood felt with 

 more success than blotters. Kling (53) recommended clay pots rather 

 than Petri dishes or blotting paper. Finally Goss (29) stated that 

 Petri dishes gave consistently low results, while the achenes germinated 

 well on or between blotters. 



All investigators (9, 28, 43, 45, 52, 69) agreed that Poa achenes germi- 

 nated poorly at constant temperatures, especially in a partly after-ripened 

 condition and in the absence of light, but that proper daily intermittent 

 temperatures were very effective. In fact, some workers (28, 43, 52, 

 95, 96) considered intermittent temperatures superior to KNO3 solutions 

 or even light in promoting the germination of achenes of this genus. 

 Toole stated that for complete germination the usual samples of Poa 

 pratensis did not require exposure to light or treatment with KNO3 

 solution in addition to intermittent temperatures, but that many samples 

 of Poa compressa required both of these in addition to intermittent 

 temperatures. Contrary to the results of Toole and Gassner, Maier 

 (71) found that light generally favored the germination of Poa achenes 

 in both intermittent and constant temperatures, and that for P. nemoralis 

 and some other Poa species the effect of light even surpassed that of alter- 

 nating temperatures. There was considerable disagreement concerning 

 the intermittent temperatures that were most effective. Toole spoke 

 of 20°C. for 18 hr. of the day and 30°C. for 6 hr. of the day as being highly 

 effective. Gassner found that the period at the lower temperature must 

 be much longer than the period at the high temperature. He obtained 

 very good results with long intermittent temperature periods, such as 

 seven days at 12°C. and one day at 24° to 28°C., and he concluded that 

 the low temperature must act about seven times as long as the high 

 temperature to get the maximum effect. This held for many combina- 

 tions, including daily intermittent temperatures. 



Many investigators (31, 32, 46, 84) found direct sunlight far more 

 effective in favoring the germination of Poa achenes than diffuse light. 

 This fact no doubt led various workers to believe that the light action was 

 due to intermittent temperatures, the high-temperature periods coming 

 at the time of illumination. Maier's recent work, however, indicated 

 that light as such has some effect, for Poa nemoralis achenes were stimu- 

 lated somewhat by a 1-min. exposure to 200 meter-candles of light, and 

 P. pratensis achenes by a few seconds of illumination. 



Kinzel (51, pages 6 and 7) studied the effect of different portions of 

 the visible spectrum and of darkness upon the germination of P. pratensis 



